Polyoxyalkylene polyether polyols have a number of uses, including, for example, their use in providing the “soft segments” of polyurethane, polyurethane/urea, and in some cases, polyester polymers. Although polyoxyethylene polyols are useful in certain applications, their higher degree of crystallinity, hydrophilicity, and hygroscopicity, restrict their use in many applications where their higher carbon content homologs, predominately polyoxypropylene and polyoxybutylene polyols, have proven satisfactory. The latter polyols, in all but the lowest molecular weight oligomers, are relatively hydrophobic, and are thus compatible with many reactive polymer systems.
Oxyalkylation of suitably hydric initiator molecules with propylene oxide, butylene oxide, and other higher alkylene oxides, results in polyoxy(higher)alkylene polyols whose terminal hydroxyl groups are largely secondary hydroxyls. Secondary hydroxyl groups are not nearly as reactive as primary hydroxyls. Because of the lower reactivity of the secondary hydroxyl group, the use of homopolymeric polyoxypropylene and polyoxybutylene polyols is rendered difficult, if not impossible, in such high volume products as high resilience (HR) polyurethane slab foam and one shot molded polyurethane foam. In less critical applications such as polyurethane elastomers and prepolymer derived foams, where high secondary hydroxyl polyols may be used in the form of prepolymers to prepare acceptable products, prepolymer processing time is extended, thereby creating an economic penalty at the raw material end rather than at the product end. Therefore, poly(higher)oxyalkylene polyols are frequently capped with polyoxyethylene groups to provide high primary hydroxyl content.
As an example, a 4200 Dalton (Da) molecular weight polyoxypropylene triol prepared by the strong base (KOH) catalyzed polyoxypropylation of glycerine may be “EO-capped” (“ethylene oxide capped”) by conducting the last portion of the oxyalkylation with ethylene oxide rather than propylene oxide. Adding enough ethylene oxide to produce a 6000 Da triol (30% EO cap) will introduce polyoxyethylene terminated molecules having substantially higher primary as compared to secondary, hydroxyl content. Unfortunately, this procedure has several drawbacks. First, because ethylene oxide is polymerized onto the molecules in random fashion, a considerable quantity of cap must be present to produce a high primary hydroxyl content. For example, a 30 wt. % ethylene oxide (EO) cap generally results in only approximately 70-80% primary hydroxyl content. Second, the large amount of polyoxyethylene content considerably alters important properties such as hydrophobicity and hygroscopicity, and may confer often unwanted surfactant properties by establishing or altering hydrophile/lipophile balance.
Low mono polyols are generally prepared by double metal cyanide complex catalyzed polyoxyalkylation. During conventional base-catalyzed oxypropylation, a competing rearrangement of propylene oxide into allyl alcohol continually generates an oxyalkylatable unsaturated mono during the course of the reaction. The polyoxyalkylation of this monomeric species produces oligomeric monols of broad molecular weight range, which not only increase polydispersity, but more importantly, decreases the product functionality. For example, polyoxypropylene triols with equivalent weights of 2000 Da may contain 40 mol percent monol, thus lowering the theoretical, or “nominal” functionality from 3.0 to the range of 2.1 to 2.3.
Double metal cyanide (DMC) complex catalysts such as non-stoichiometric zinc hexacyanocobaltate glyme complexes are able to produce polyoxypropylene polyols with low monol content, as reflected by levels of unsaturation of 0.015 to 0.020 meq/g polyol, compared to unsaturation of 0.06 meq/g to 0.012 meg/g in moderate to high molecular weight, conventionally base-catalyzed polyols. Improvements in DMC catalysts, described in U.S. Pat. Nos. 5,470,813 and 5,482,908, permit production of “ultra-low” unsaturation polyols, with unsaturation in the range of 0.003 meq/g or lower, to about 0.010 meq/g. The monol content of polyols produced by these catalysts is exceptionally low, in the worst cases about 2 mol percent, and often virtually unmeasurable. Moreover, the polydispersity of these polyols is exceptionally low. The polyols, in many cases, are essentially monodisperse.
Ethylene oxide (EO) capping of DMC catalyzed polyoxy(higher)alkylene polyols may be accomplished by purifying the polyol to remove the DMC catalyst, followed by addition of another oxyalkylation catalyst. Another technique is to denature the DMC catalyst by adding an excess of strong alkali metal base, the excess serving as the polyoxyethylation catalyst. However, these methods require adsorption, neutralization, and/or filtration of the generally rather viscous polyol product to remove basic catalyst residues, increasing cost of the product. Moreover, the polyoxyethylene capping (“EO capping”) suffers from the same drawbacks as the ethylene oxide (EO) capping of conventional, base-catalyzed polyols: the capping is both inefficient and further alters the physical characteristics of the polyol. It is especially difficult to prepare high primary hydroxyl polyoxy(higher)alkylene polyols with low molecular weights without grossly altering polyol properties.
Therefore, a need exists in the art for a DMC-catalyzed process for producing polyoxyalkylene polyols having a relatively high primary hydroxyl content without significantly altering other polyol properties. It is furthermore desirable to develop a process for directly preparing such DMC-catalyzed polyols requiring a greatly reduced amount of capping than is presently needed.